1. Field of the Invention
The present invention relates to the preparation of carboxylic acids, and, more especially, to the preparation of carboxylic acids, notably acetic acid, by carbonylation of an alcohol.
2. Description of the Prior Art
It is well known to this art that, e.g., acetic acid, can be prepared by carbonylation of methanol under relatively severe reaction conditions of pressure, in the presence of nickel and a free or bound halogen.
Thus, it has been proposed, in particular (compare U.S. Pat. No. 2,729,651), to carry out the carbonylation of methanol in the presence of nickel complexes which are obtained by reacting nickel halides with quaternary ammonium (or phosphonium) halides, i.e., complexes of the general formula: EQU [A.sub.4 M].sub.2 NiX.sub.4
in which X represents a bromine or iodine atom, M represents a phosphorus or nitrogen atom and A is, for example, a lower alkyl radical.
These complexes can be used in the aforesaid form in the reaction in question, or they can be formed in situ. However, although increased pressure (on the order of 700 atmospheres) is applied during the carbonylation reaction, the efficiency of the catalyst system, expressed in terms of hourly output or yield, is very low.
It has been possible to substantially improve this output, expressed either with respect to the reaction volume or with respect to the amount of nickel used, by employing in the subject reaction a nickel halide, on the one hand, and, on the other hand, a quaternary ammonium (or phosphonium) halide in a quantity greater than that required by the stoichiometry of formation of the complexes of the aforenoted formula. Compare German Pat. No. 933,148. However, in this latter case, the pressure conditions remain severe.
More recently, catalyst systems which permit the carbonylation of methanol under less severe conditions of pressure have been proposed. Thus, published French Patent Application No. 2,370,023 describes the carbonylation of methanol in the presence of at least 10 mols of methyl iodide per 100 mols of methanol, and in the presence of nickel and of a free phosphine and/or a phosphine complexed with the nickel, under a pressure of less than 70 bars. However, the efficiency of such a system, again expressed in terms of hourly output, remains low.
In the context of a carbonylation process under low pressure, it has also been emphasized (compare published French Patent Application No. 2,404,618) that the presence of solvents, such as carboxylic acids or their esters, has a favorable effect on the course of the carbonylation reaction. However, utilization of such solvents is not essential, and the starting material alcohol can also serve as the solvent.
However, the industrial or commercial value of these recent developments is limited as a result of the instability and the cost of the phosphines or of the amines required to effect these techniques.